The description of the present invention that follows is directed to both a tubular molded product and a columnar molded product. Since techniques for molding a tubular product are more complicated, However, when the words "tubular (columnar)" appear in the following description, it is intended to refer to both a tubular molded product and a columnar molded product.
Molded products consisting of a thermoplastic resin have been utilized as replacements for metal products since the former can be made relatively easily and efficiently by injection molding techniques.
However, the dimensional accuracy of an injection molded resin product is generally inferior to that of product produced by cutting or shading a piece of stock. serious problems can thus arise in many cases in a tubular (columnar) molded product which demands, due to its intended end-use application, high dimensional accuracy, and especially roundness (magnitude of roundness deviation of an actual product from that of a geometrically correct circle, and errors of various diameters of a predetermined circular cross section of an actual product) and cylindricity (magnitude of cylindrical deviation of an actual product from that of a geometrically correct cylinder, and differences between the sizes (diameters) of various circular cross sections of a tube (column)).
On the other hand, the dimensional accuracy of a tubular molded product to be obtained by conventional injection molding methods is improved generally by regulating the temperature of the metal mold to as uniform a level as possible. In this regard, attempts to improve the dimensional accuracy of injection molded products have included regulating the temperature of the product core (which easily receives and accumulates therein the heat radiated from the resin, and which constitutes the inner hollow of a final tubular product) with a heat pipe or cooling air.
Even when such a conventional technique is used, a product core in which the heat may be accumulated is cooled uniformly, so as to minimize and change the occurrence of voids in the resin and resin shrinkage. This enables a molded product of a comparatively high accuracy to be obtained. Although this method is considerably effective, especially in improving the roundness which is one of the important accuracy determining elements for a tubular molded product, it is substantially ineffective in improving the other important accuracy determining element i.e., cylindricity. This method is not capable of preventing a phenomenon in which the outer diameter of an end portion of a tubular molded product becomes larger than that of an intermediate portion thereof. More specifically, a tubular molded product obtained by this method becomes divergent at the outer circumferences of the end portions thereof as shown in FIG. 5C. Even in the case where the temperature of a metal mold is regulated so as to attain a uniform level and improve the accuracy of a final columnar molded product, it is inevitable that the a product thereby obtained will be deformed due to the shrinkage of an intermediate portion thereof. Moreover, such a product will be divergent at the circumferential end portions thereof in the same manner as the tubular molded product referred to above. The presently available injection molding technique does not meet in many cases the requirements for the production of a tubular (columnar) part having a high dimensional accuracy, and especially, a high cylindricity. In such cases, tubular (columnar) products obtained by injection molding are thereafter subjected to further processes, for example, which thereby results in higher manufacturing costs. Thus, what has been needed are improved and economically advantages cutting techniques for producing a tubular (columnar) molded product having high dimensional accuracy.
The present invention is directed to a metal mold for injection molding tubular (columnar) products, characterized in that the portions of matching surfaces of metal mold members which are close to end portions of an annular (cylindrical) cavity are provided with grooves extending continuously or intermittently so as to surround the end portions of the annular (cylindrical) cavity. The invention also relates to a tubular (columnar) molded product of a thermoplastic resin molded by this metal mold.